Effects of N 2 O elimination on the elimination of second gases in a two-step mathematical model of heterogeneous gas exchange.

We have investigated the elimination of inert gases in the lung during the elimination of nitrous oxide (N 2 O) using a two-step mathematical model that allows the contribution from net gas volume expansion, which occurs in Step 2, to be separated from other factors. When a second inert gas is used in addition to N 2 O, the effect on that gas appears as an extra volume of the gas eliminated in association with the dilution produced by N 2 O washout in Step 2. We first considered the effect of elimination in a single gas-exchanging unit under steady-state conditions and then extended our analysis to a lung having a log-normal distribution of ventilation and perfusion. A further increase in inert gas elimination was demonstrated with gases of low solubility in the presence of the increased ventilation-perfusion mismatch that is known to occur during anesthesia. These effects are transient because N 2 O elimination depletes the input of that gas from mixed venous blood to the lung, thereby rapidly reducing the magnitude of the diluting action.